Research into various materials used in photovoltaic devices in addition to silicon in order to reduce the manufacturing cost and increase the efficiency has been carried out. However, because of the properties of the photovoltaic devices that use the semiconductor principle, these devices have much lower efficiency and a shorter lifespan due to degradation, compared to silicon based photovoltaic devices, and thus the actual market occupation rate thereof is as low as about 3%.
In the silicon based photovoltaic devices, silicon monocrystals or silicon polycrystals are mainly used, and upon construction of a photovoltaic system, the silicon material and wafer costs are more than 40% of the total construction cost. With the goal of substantially solving this problem, a lot of effort is being directed to reducing the amount of silicon necessary for unit power production through morphology/engineering approach and to minimizing consumption of silicon in thin-film type devices.
In order to reduce the amount of silicon necessary for unit power production, the quantity of light absorbed per mass of silicon should be increased, the electron-hole pair production efficiency should be increased upon light absorption, and the produced electron-hole pair should be prevented from re-combining and should be effectively separated.
In the case of conventional horizontal junction photovoltaic devices in which the p-n or p-i-n junction is parallel to the substrate, the light absorbing layer of the photovoltaic device is confined to the depletion layer or neutral layer (i) having a planar structure formed on the p-n junction. In the conventional horizontal junction photovoltaic device, when the light absorbing layer (the p-n depletion layer, or the i layer) is formed thick, a large amount of light may be absorbed, but the distance between opposite electrodes of the junction may increase, undesirably reducing the electric field, making it difficult to separate the electron-hole pairs. Hence, limitations are imposed on increasing the thickness of the junction. Also, the light wavelength range over which light is able to be absorbed becomes very narrow, and so the theoretical conversion efficiency is known to be less than 30%.